Among these free parasites, many do not attach themselves to a particular species, and well deserve the title of cosmopolitan parasites. Thus we see that the _Ascaris lumbricoides_, so common among children, lodges also in the ox, or the horse, the ass, and the pig. The _Distoma hepaticum_, which is a parasite peculiar to the sheep, if we may judge by its abundance in this animal, may find its 96 way into the liver of man, or into that of the hare, the rabbit, the horse, the squirrel, the ass, the pig, the ox, the stag, the roebuck, and different species of antelope. It is to be remarked that all these animals have a vegetable regimen. By drinking the water which contains the cercaria of this species, they grow infested by this singular lodger. The large Echinorhyncus (_E. Gigas_) has been found in the dog, and the pig, perhaps in the phocinæ; and instances are mentioned in which it has even migrated into man. The _Gordius aquaticus_ appears to live and develop itself in different species of insects; and among the articulated parasites, we meet with the _Ixodes ricinus_, commonly called the tick, on the dog, the sheep, the roebuck, and the hedgehog; and instances are given of its presence on man. It has been long since proved in menageries and zoological gardens, that the _Acarus_ of the camel is able to give a cutaneous disease to man. As we have before said, there are many parasites which require to be studied in order to determine the host peculiar to each of them; although parasites sometimes lose their way, and introduce themselves into the wrong neighbour, yet they can live there but a short time. Instances have been known, in which the larvæ of flies have penetrated into man accidentally by the mouth or the nostrils. Reptiles have been known to live a certain time in the stomach. A German physiologist, Berthold, professor at the University of Göttingen, has given an account of all those which have been found under such circumstances, and the number of them is considerable; he has written a memoir on the abode of living reptiles in man. Among other instances, this naturalist mentions the case of a boy of 97 twelve years of age, who, in 1699, after suffering acute pain, voided from the intestines nearly one hundred and sixty four millipedes, four scolopendræ, two living butterflies, two worm-like ants, thirty-two brown caterpillars of different sizes, and a coleopterous insect. These animals lived from three to twelve days. This is not all: the same child, two months afterwards, voided four frogs, then several toads, and twenty-one lizards, and sometimes a live serpent was seen for a moment at the bottom of his mouth. Happily for science, we do not see such things seriously related in books at the present day.